The present invention generally relates to the preparation of semiconductor grade single crystal silicon which is used in the manufacture of electronic components. More particularly, the present invention relates to a process for producing a single crystal silicon ingot by the Czochralski method, a substantial portion of which is substantially free of agglomerated intrinsic point defects, wherein the crystal growth conditions are varied to initially create alternating regions of silicon self-interstitial dominated material and vacancy dominated material within the constant diameter portion of the ingot. The regions of vacancy dominated material act as sinks to which self-interstitials may diffuse and be annihilated.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski (“Cz”) method. In this method, polycrystalline silicon (“polysilicon”) is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon and a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the growth rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter is reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the crystal growth rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt.
In recent years, it has been recognized that a number of defects in single crystal silicon form in the crystal growth chamber as the crystal cools after solidification. Such defects arise, in part, due to the presence of an excess (i.e. a concentration above the solubility limit) of intrinsic point defects, which are known as vacancies and self-interstitials. Silicon crystals grown from a melt are typically grown with an excess of one or the other type of intrinsic point defect, either crystal lattice vacancies (“V”) or silicon self-interstitials (“I”). It has been suggested that the type and initial concentration of these point defects in the silicon are determined at the time of solidification and, if these concentrations reach a level of critical supersaturation in the system and the mobility of the point defects is sufficiently high, a reaction, or an agglomeration event, will likely occur. Agglomerated intrinsic point defects in silicon can severely impact the yield potential of the material in the production of complex and highly integrated circuits.
Vacancy-type defects are recognized to be the origin of such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques such as Scanning Infrared Microscopy and Laser Scanning Tomography. Also present in regions of excess vacancies are defects which act as the nuclei for ring oxidation induced stacking faults (OISF). It is speculated that this particular defect is a high temperature nucleated oxygen agglomerate catalyzed by the presence of excess vacancies.
Defects relating to self-interstitials are less well studied. They are generally regarded as being low densities of interstitial-type dislocation loops or networks. Such defects are not responsible for gate oxide integrity failures, an important wafer performance criterion, but they are widely recognized to be the cause of other types of device failures usually associated with current leakage problems.
The density of such vacancy and self-interstitial agglomerated defects in Czochralski silicon is conventionally within the range of about 1*103/cm3 to about 1*107/cm3. While these values are relatively low, agglomerated intrinsic point defects are of rapidly increasing importance to device manufacturers and, in fact, are now seen as yield-limiting factors in device fabrication processes.
One approach which has been suggested to control the formation of agglomerated defects is to control the initial concentration of the point defects when the single crystal silicon is formed upon solidification from a molten silicon mass by controlling the growth rate (v) of the single crystal silicon ingot from the molten silicon mass, wherein higher growth rates tend to produce vacancy rich material and lower growth rates tend to produce interstitial rich material, and controlling the axial temperature gradient, G, in the vicinity of the solid-liquid interface of the growing crystal for a given temperature gradient. In particular, it has been suggested that the radial variation of the axial temperature gradient be no greater than 5° C./cm. or less. See, e.g., Iida et al., EP0890662. This approach, however, requires rigorous design and control of the hot zone of a crystal puller.
Another approach which has been suggested to control the formation of agglomerated defects is to control the initial concentration of vacancy or interstitial point defects when the single crystal silicon is formed upon solidification from a molten silicon mass and controlling the cooling rate of the crystal from the temperature of solidification to a temperature of about 1,050° C. to permit the radial diffusion of silicon self-interstitial atoms or vacancies towards the lateral surface of the ingot or towards each other causing recombination thereby suppressing the concentration of intrinsic point defects to maintain the supersaturation of the vacancy system or the interstitial system at values which are less than those at which agglomeration reactions occur. See, for example, Falster et al., U.S. Pat. No. 5,919,302 and Falster et al., WO 98/45509. While these approaches may be successfully used to prepare single crystal silicon which is substantially free of agglomerated vacancy or interstitial defects, significant time may be required to allow for adequate diffusion of vacancies and interstitials. This may have the effect of reducing the throughput for the crystal puller.